Recently, an absorbent resin is widely used in sanitary goods such as paper diaper's, sanitary napkins, etc., for the purpose of absorbing liquid.
Examples of the conventionally known absorbent resins includes: a partially neutralized crosslinked polymer of polyacrylic acid (Japanese Unexamined Patent Publication No. 84304/1980 (Tokukaisho 55-84304), Japanese Unexamined Patent Publication No. 108407/1980 (Tokukaisho 55-108407) and Japanese Unexamined Patent Publication No. 133413/1980 (Tokukaisho 55-133413)); a hydrolyzed polymer of starch-acrylonitrile (Japanese Unexamined Patent Publication No. 43995/1971 (Tokukaisho 46-43995)); a neutralized graft polymer of starch-acrylic acid (Japanese Unexamined Patent Publication No. 125468/1976 (Tokukaisho 51-125468)); a saponified copolymer of vinyl acetate-acrylic ester (Japanese Unexamined Patent Publication No. 14689/1977 (Tokukaisho 52-14689)); a hydrolyzed copolymer of acrylonitrile or of acrylamide (Japanese Unexamined Patent Publication No. 15959/1978 (Tokukaisho 53-15959)); a crosslinked polymer of cationic monomer (Japanese Unexamined Patent Publication No. 154709/1983 (Tokukaisho 53-154709) and Japanese Unexamined Patent Publication No. 154710/1983 (Tokukaisho 58-154710)).
Among the above-listed crosslinked polymers, a crosslinked polymer obtained by polymerizing a hydrophilic unsaturated monomer having at least one carboxyl group such as acrylic acid in the presence of a crosslinking agent is the most common material for the absorbent resin, because a raw material for the above polymer is available at low price, while exhibiting excellent absorbent properties without the problem of putrefaction.
The absorbent resin desirably exhibits the following properties when it is in contact with an aqueous liquid: high magnifier and rate of absorbency; an excellent stability of its swollen gel; high diffusivity of and permeability to aqueous liquid, a strong suction power for sucking the aqueous liquid from the material containing aqueous liquid, etc.
Recently, still thinner sanitary goods of higher quality have been demanded, and an amount of use of the absorbent resin has been increasing. Thus, an absorbent resin which exhibits improved gel stability, diffusivity of and permeability to aqueous liquid is strongly demanded. As well known, these properties of the absorbent resin are greatly affected by a crosslinking agent used in the preparation process of the absorbent resin.
The crosslinking agent used in the preparation of the absorbent resin must be commercially available and safe, and it is also required to exhibit excellent affinity and polymerizability with a hydrophilic unsaturated monomer. For the above mentioned absorbent resin, unsaturated carboxylates of polyhydroxy alcohol, which have a plurality of polymerizable unsaturated groups and are polymerizable with a hydrophilic unsaturated monomer having at least one carboxyl group may be used. Examples of the above unsaturated carboxylates include: di(meth) acrylate of polyoxyalkylene glycol, (poly)propylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexamethacrylate, and the like.
In order to prepare the absorbent resin which exhibits an excellent gel stability and diffusivity of aqueous liquid using one of the above-listed unsaturated carboxylates as a crosslinking agent, the crosslinking agent must be used in a large amount. Here, the amount of use of the crosslinking agent and the gel stability have a virtually positive relationship. However, because of other factors, such as copolymerizability, affinity and solubility of the crosslinking agent and the hydrophilic unsaturated monomer having at least one carboxyl group, etc., the reactivity of the crosslinking agent is lowered when it is used in an excess amount of a predetermined range. Namely, the excessive amount of the crosslinking agent would not give an improvement in the above properties of the absorbent resin as much as expected for the amount of use of the crosslinking agent. Moreover, the effect of reducing the water soluble polymer content, which affects the diffusivity of and permeability to aqueous liquid of the absorbent resin, is reduced. Thus, in order to obtain absorbent resin which exhibits an excellent gel stability, a large amount of the crosslinking agent is required.
In order to improve the copolymerizability, affinity and solubility of the crosslinking agent and the hydrophilic unsaturated monomer, for example, a method for polymerizing in the presence of a surface active agent (Japanese Examined Patent Publication No. 36763/1986 (Tokukosho 61-36763)) and a method for polymerizing in the presence of a dispersant (Japanese Unexamined Patent Publication No. 146902/1989 (Tokukohei 1-146902) are disclosed. However, since all of the above methods present the problem that the surface active agent or the dispersant remains in the resulting absorbent resin, which lowers the absorbent properties of the resulting absorbent resin, they are not desirable.